In recent years, there has been a major upswing in the use of cathodic sputtering as a coating technique. Sputtering processes have become increasingly diversified and intrinsically more refined. The use of sputtering-dependent coating techniques ranging from the production of wear-resistant and/or corrosion-resistant surface bodies, layer by layer, is a particularly common practice in the electronics industry. Such targets are subject to wide and varied requirements, not only as to the composition, but also as to the shape and physical condition of the resultant target.
Although for a long time only individual metals and commonly-used metal alloys were employed as target materials, recently there has been a demand for targets which permit several metals to be sputtered at the same time, typically in closely-toleranced mixture ratios and at high degrees of purity, even if the different metals used are incapable of or unsuitable for alloying, or if both metallic and non-metallic components are involved. Metals or metal/non-metal compounds which are generally considered incapable or unsuitable for alloying include, among others, individual components which are mutually immiscible in the liquid phase, and/or which cannot be induced to solidify homogeneously from the metal due, in part, to dissimilar densities.
Brittle intermetallic phases often occur in many alloy systems presently desired for use as sputter targets. Solid bodies of these compositions are neither workable (either by cold-working or hot-working) nor machinable by conventional chip-forming methods, except possibly at great expense and effort. Cathodic sputtering units currently in use are also of many different designs. These designs call for targets of many specific and different shapes which are often geometrically very complex. As a rule, these shapes are only producible by conventional machining from blanks. For conductivity and stability reasons sputter targets frequently have to be mounted on supporting materials, typically with screws. This calls for the provision of additional holes in the target which are, at best, difficult to form in brittle materials.
Target breakage is often the cause of work stoppage when coating units are operated industrially. Even the act of fixing a screw in a marginally ductile target to attach it to a supporting material sometimes causes immediate breakage. This is because, in part, the screw must be fastened very tightly to insure intimate contact and good thermal and electrical conductivity in the contact area. Temperature differences between the base cathode and the target can also lead to additional mechanical stresses in the target. This is also a common cause of breakage due to stresses created by non-uniform heating of the target because of local variations in the intensity of ion bombardment during operation.
The requirement for a high density or, rather, a high degree of freedom from porosity renders target manufacture more difficult, and has precluded the use of many metallurgical and powder-metallurgical techniques.
German Pat. Document No. DE A1 24 26 922 gives an overview of the many difficulties that can arise in the "manufacture of solid, homogeneous workpieces from constitutionally complex alloys". As described in that patent, workpieces composed of complex alloys including, for example, targets, are often produced by hot-pressing homogeneous powder mixtures composed of the individual alloy components. This technique involves compacting the components under multi-directional pressure conditions, particularly at isostatic conditions, and is generally accompanied by fusion of the component having the lowest melting point, and the formation of bodies having rather simple geometric target shapes. If a suitable alloy forms brittle intermetallic phases, hot-pressed shaped bodies therefrom will be brittle as well, and will only be able to withstand light mechanical stresses, making the use of these materials unsuitable for cathodic sputtering targets. Moreover, when the hot-pressing manufacturing technique is used, it is impossible or nearly impossible in many cases to achieve the degree of freedom from porosity desired for target materials.
German Pat. Nos. DE A1 31 49 910 and DE C2 29 40 369 describe another method for manufacturing multicomponent targets not amenable to processing into ductile alloys, but nevertheless used in cathodic sputtering units. In the method of these patents, blind holes are made in the parent target material and the sputter target is provided with plugs which can be press-fitted into the blind holes. The same procedure can be followed if additional target materials are required. However, this method is comparatively expensive and, because of different thermal expansions of the individual components, the plugs can loose the secure fit, and hence loose the thermal and electrical conductivity required with the parent target. Moreover, given the local distances between the various materials that are to be sputtered, and given the generally dissimilar sputtering rates at different zones on the surface of a target, it is often impossible to assure that different components will be sputtered at rates which yield constant and acceptable concentration ratios required by the user.